A strange epidemic is sweeping the Riviera. In desperation the French authorities call upon Dr Petrie to find an answer. During this crisis, a mysterious siren on the beach captivates Alan Sterling. She tells him her name only - Fleurette - and flees. When Petrie's lab cultures show up sleeping sickness and plague, they call in Sir Denis Nayland Smith. It is not long before their investigations lead them to Fleurette - and to Dr Fu Manchu.